1. Field of the Invention
The present invention relates to an improved method and apparatus for sterilizing liquid food products through the use of steam infusion.
2. Description of the Prior Art
It has long been recognized that certain food products lend themselves to preservation by the use of heat to sterilize the product. Sterilization destroys bacteria and inactivates most enzymes which cause product spoilage. Additionally sterilization of certain food products may eliminate the need for refrigeration. This is particularly true in the case of dairy products such as milk or ice cream mix, to which this invention is primarily directed, although it may be equally applied to other liquid products such as juices.
In the sterilization of milk, it is necessary to raise the temperature of the milk sufficiently to destroy all bacteria and inactive enzymes which causes spoilage. The rate of destruction or inactivation of these organisms varies with both temperature and the time during which the product is held at an elevated temperature. A preferred method of sterilizing milk and dairy products has been to utilize steam infusion to subject the milk to ultra high temperatures for very short periods of time followed by flash cooling. This has been proven to achieve superior product flavor. Various approaches have been used in the past to accomplish this. For example U.S. Pat. No. 3,156,176 to Wakeman describes a heating apparatus in which steam is supplied into a chamber with the liquid product being introduced in the form of a curtain-like film to expose the fluent product to the elevated steam temperatures. Similarly, U.S. Pat. No. 2,899,320 to Davies and U.S. Pat. No. 3,032,423 to Evans, both utilize apparatus for containing steam in which the product is passed over plates within the steam chamber and heated while the product flows downwardly to a collection point for delivery to a flash chamber. A variation of this method is also described in U.S. Pat. No. 3,771,434 to Davies in which screen panels are used to form a thin film of product for exposure to steam. One major disadvantage of the methods and apparatus described in the foregoing patents is the fact that liquid food products, particularly milk products, have a tendency to burn and collect on heated surfaces which are at temperatures greater than or equal to the temperature of the product itself. Such burning, in addition to fouling the apparatus itself necessitating periodic cleaning, also results in undesirable flavor changes to the milk product.
In an obvious effort to avoid such burn-on and fouling, U.S. Pat. No. 4,310,476 to Nahra and U.S. Pat. No. 4,375,185 to Mencacci attempt to form free falling thin films of milk within a steam atmosphere for raising the product temperature. A problem associated with attempting to form a free falling thin film is that the integrity of such films is very unstable and are subject to splashing or break-up in the presence of moving or circulating steam and gases. Film formation requires close adherence to flow parameters and such devices are also subject to the product burn-on problems when hot surfaces are contacted. Additionally, it is recognized, as discussed in the Nahra patent that physical agitation of milk may also affect the ultimate flavor of the treated product and disturbance of the free falling films will result in such agitation.
Another problem associated with many of the prior art approaches to steam infusion of liquid products is that the devices are not easily cleaned for example with the use of clean-in-place systems. The more internal components in which the product may collect or burn-on, the more difficult the cleaning process.
A further problem which has been recognized in the steam infusion processes is dealing with the presence of entrained gases released from the product when heated and gases, air or oxygen remaining after the steam condenses into the product. As noted by A. G. Perkin, "Review of UHT Processing Methods" journal of the Society of Dairy Technology, Vol. 38, No. 3, July, 1985, gases trapped within the pressure vessel result in increased pressures and greater temperature differences between the steam and product which can again increase the risk of overheating the product and undesirable flavor changes.
Finally, in an effort to reduce burn-on, U.S. Pat. No. 3,771,434 to Davies and the patent to Nahra both suggest the use of cooling mediums in the lower levels of the chambers where the product is collected. The use of cooling jackets, however, reduces the efficiency of the steam chamber by increasing the internal operating pressure and amount of condensation of steam thereby requiring additional quantities of higher temperature steam to make up for such losses. This directly effects the efficiency of the process as well as the flavor of the end product.